FIG. 1 graphically illustrates a simplified prior art system 48 for identifying RFID tags 56 in a high tag density environment having unequally spaced RFID tags 56 using isotropic transmit and receive antennas 50 and 52. The transmit antenna 50 emits an RF wavefront 54 which acts as an RF interrogation signal 54. This interrogation signal 54 causes all the RFID tags 56 within the transmitting radius to respond by sending an encoded RF signal with the tag's ID number and optionally other data stored on the tag 56. The receive antenna 52 then receives this encoded signal response from the tag 56 and sends it on to the rest of the system 48. The receive antenna 52 may be separate from or part of the structure containing the transmit antenna 50. When multiple tags all transmit at the same time, they interfere with each other causing read errors, which in turn causes many retransmits to occur. In the worst-case, the number of successful reads goes to zero.
Because the FCC limits the amount of power that may be transmitted, and because in many systems the tags are passive, i.e. they rely on the transmitted RF power to obtain the power they require for operation and retransmission of tag information, the range is quite limited if the antenna structures are isotropic. In order to cover large geographic areas, this forces a large number of reader/transmitters. Therefore a need exists to replace isotropic antennas with directional antennas to solve these and other problems when reading RFID tags in a high-density environment.